Typically, during semiconductor processing, a reactive ion etch (RIE) process is employed for etching fine line patterns in a silicon wafer. Reactive ion etching involves positioning a masked wafer in a chamber which contains plasma. The plasma contains etchant gases which are vertically dissociated in an RF field so that reactive ions contained in the etchant gases are accelerated to the wafer surface. The accelerated reactive ions combine chemically with unmasked material on the wafer surface. As a result, volatile etch product is produced. During such etching, a single layer or multiple layers of material or film may be removed. Such material includes, for example, SiO.sub.2, poly-silicon, and silicon nitride.
As a film of unmasked material is etched, the volatile etch product is incorporated into the plasma. As the RIE process approaches the interface or end of the film layer being etched, the amount of volatile etch product found in the plasma decreases since the amount of unmasked material being etched is reduced due to the etching. Oftentimes, the amount of volatile etch product in the plasma is tracked in order to determine the end-point of the RIE process, i.e., depletion or decrease in the amount of volatile etch product in the plasma during the RIE process can be used as an indication of depletion of unmasked material being etched for ending the process.
It is also possible to track a reactive species, i.e., one of the etchant or input gasses used to etch the film. As the film is etched, the reactive species will be depleted and relatively low concentrations of the reactive species will be found in the plasma. However, as the film is consumed, the reactive species will be found in the plasma in increasingly higher concentration. A time trace of the optical emission from such a reactive species will show an increase in intensity as the film is etched off.
End-point detection refers to control of an etch step and is therefore an important issue in RIE processes. Tracking the intensity of a wavelength for a particular species using optical emission spectroscopy (OES) is often used for end-point control of a RIE process.
Frequently, OES is used to track the amount of either volatile etch product or reactive species as a function of film thickness. This technique examines emissions from either the volatile etch product or reactive species in the plasma. As the film interface is reached during etching, the emission species related to the etch of the film will either decrease in the case of volatile etch product or increase in the case of reactive species.
More specifically during a RIE process, plasma discharge, i.e., etchant, neutral, and reactive ions in the plasma, is continuously excited by electrons and collisions, thus giving off emissions ranging from ultraviolet to infrared radiation. An optical emission spectrometer diffracts this light into its component wavelengths. Since each species emits light at a wavelength characteristic only of that species, it is possible to associate a certain wavelength with a particular species, and to use this information to detect etch end-point.
As an example, in etching SiO.sub.2 with CHF.sub.3, carbon combines with oxygen from the wafer to form carbon monoxide as an etch product. It is known that carbon monoxide emits light at a wavelength of 451 nm, and that this wavelength can be monitored for accurately detecting the end-point for such an etch. When the oxide is completely etched there is no longer a source of oxygen and the CO peak at 451 nm decreases, thus signaling end-point.
In the above example, it is known that light emitted from CO at a wavelength of 451 nm should be used for end-point detection. However, such information is generally unavailable, and it has been found to be a formidable task to determine or select the wavelength to use for accurate end-point control. This difficulty exists because of the numerous possibilities for emissions. In other words, any molecule may emit light at a multitude of different wavelengths due to the many transition states available for de-excitation. Therefore, given the process, gases utilized, and the material being etched, it is typically not readily known which wavelength in the spectrum to monitor for end-point control. In this regard, the OES spectrum for a typical RIE etch is composed of hundreds of wavelengths in the visible and ultra-violet bands.
Additionally, there is a present day trend towards using high density plasma sources to replace reactive ion etching. One example is electron cyclotron resonance (ECR), which differs from reactive ion etching in plasma formation. Generally, electron cyclotron resonance operates at a lower pressure than a conventional RIE system, and is therefore able to etch finer-line trenches anisotropically. Comparison studies of the emissions from ECR and RIE plasmas show emphasis on different species and different wavelengths for the same input gas composition. The excitation mechanisms and interactions of the particles at lower pressure account for these differences. The consequence of this for end-point is that the experience and knowledge accumulated from RIE emissions may not carry over to ECR eissions. In other words, it may not be possible to monitor the same species or wavelength for end-point detection in ECR as was monitored for RIE, even if the same material is being etched using the same input gas composition.
Thus, there remains a need in present day and future technology for optimizing selection of a wavelength to monitor for end-point detection during etching.